Final answer:
About 21% of air is composed of oxygen, which is crucial for most forms of life and is the second most prevalent gas in the atmosphere after nitrogen.
Step-by-step explanation:
The percentage of oxygen in the air is approximately 21%. This vital gas is needed by most organisms for respiration and energy production. In the Earth's atmosphere, the major component is nitrogen (approximately 78%), with oxygen being the second most prevalent. For environments where oxygen levels are low, such as high-altitude areas or in certain work environments, oxygen can be supplied from tanks containing the compressed gas. Understanding the composition of air and the partial pressures of its gases is important because each gas exerts its own pressure. The partial pressure of a gas is directly proportional to its percentage in a mixture, a concept vital in fields like respiratory medicine and engineering.